1. Field of the Invention
The present invention relates to artificial joints, and more particularly to ball and socket replacement joints for human beings. While the invention is applicable to a wide range of artificial joints, it is especially suitable for and will be described particularly in connection with an artificial femoral head that may be needed for replacement due to various reasons, such as fracture or osteoarthritis of the head or neck of the femur. It will also be described in connection with an artificial hip joint that may be needed for replacement of the roof of the acetabulum and the head of the femur for various reasons, such as fracture of the head of the femur, or osteoarthritis of the roof of the acetabulum.
2. Description of the Related Art
The construction of a conventional artificial femoral head is shown in FIG. 1. The head is of a dual structure consisting of an inner ball 17 and an outer ball 15. A slide bearing 16 made of a resin, such as high-density polyethylene, is fitted between the two balls. The inner ball 17 is fitted into the slide bearing 16 and secured with a fixing ring. Both the inner and outer rings are commonly made of metals such as titanium alloys, stainless steel, and CO-Cr-Mo alloys. Attempts have recently been made to form the inner and outer balls of ceramics, which have been commercialized in some areas. However, the slide bearing is still made of a resin and hence has poor wearing characteristics (i.e., wears rapidly).
In most of the conventional artificial femoral heads having the above-described dual structure, the bearing portion is of a slide type made of a high-density polyethylene, and both the inner and outer balls are made of a metal. However, the resin bearing does not have high wear resistance if the outer surface of the inner metallic ball deteriorates for some reason such as corrosion. As a result the slide bearing will become loose, potentially causing a complication such as bone fracture.
Conventional artificial hip joints as shown in FIGS. 16 and 17 comprise an acetabular socket 21, a slide bearing 38 formed of a high-density polyethylene, and an artificial femoral head 22 which is inserted into the socket so that it contacts the inner surface 35 of the bearing 38. The outer surface of the slide bearing may be coated with a metal reinforcement 36 that will prevent deformation of the bearing. In current commercial products of artificial hip joints, threads are cut in the outer surface of the socket so that the joint can be screwed into place. In a bone cement fixing system, the outer surface of the socket is roughened so as to insure good adhesion to the bone cement.
If the hip joint described above is used in the human body for an extended period, the surface of the femoral head which is typically made of a metal will corrode and its surface become so coarse that the bearing portion will wear to an abnormal extent. Attempts have recently been made to use ceramics in the femoral head, which have been commercialized in some areas. With such ceramic femoral heads, the bearing portion will not wear upon contact with the femoral head. However, during prolonged use, the bearing portion will be deformed by cyclic application of load and if the deformation proceeds further, the femoral head will penetrate the bearing portion, making the artificial hip joint no longer suitable for use.